Method for Treating Sludge of the Sewage Treatment Plants by Using Circulating Fluidized Bed Combustion

ABSTRACT

The present invention discloses a method for treating sludge of the sewage treatment plants by using circulating fluidized bed boiler combustion. The method includes the following steps: (1) Preparing step of sludge coal mixture by adding fine coal and dispersant into sludge from the sewage treatment plants, then stirring and adding stabilizing agent; (2) clean burning step of the sludge coal mixture by feeding the prepared sludge coal mixture into the circulating fluidized bed boiler for clean burning, wherein a furnace of the circulating fluidized bed boiler contains fluidized media of quartz sand and limestone. The preparation of the sludge coal mixture also includes the steps: firstly, waste water is adsorbed and filtered by fine coal and CaO to form the resulted coal sludge into which others fine coal is added, thus the sludge coal mixture is prepared.

FIELD OF THE INVENTION

This invention relates to a method for treating sewage and sludge, especially a method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion for the purpose of harmless treatment and comprehensive utilization of the sludge produced from the urban sewage treatment plants.

BACKGROUND TO THE INVENTION

A large number of sludge is produced during the processing of the urban sewage, resulting in difficult problem to the sewage treatment plants to treat the sludge.

SUMMARY OF THE INVENTION

With respect of the problem above mentioned, an object of the present invention is to provide a method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion for the purpose of harmless treatment and comprehensive utilization of the sludge produced from the urban sewage treatment plants and thus favorableness for the environmental protection.

The technical scheme by which this invention can solve the problem and reach the invention purpose is to provide a method for treating sludge of the sewage treatment plants by using circulating fluidized bed boiler combustion, comprising:

a. preparing step of sludge coal mixture by adding fine coal and dispersant into sludge from a sewage treatment plants, then stirring and adding stabilizing agent; and

b. clean burning step of the sludge coal mixture by feeding the prepared sludge coal mixture into a circulating fluidized bed boiler for clean burning, wherein a furnace of the circulating fluidized bed boiler contains fluidized media of quartz sand and limestone.

In said preparing steps of the sludge coal mixture of the method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion, the part by weight of the components of the sludge coal mixture is such as 10-20 parts for sludge, 50-60 parts for fine coal, 20-25 parts for water and 2-5 parts for dispersant and the fine coal is 50-150 μm in diameter.

In said preparing steps of the sludge coal mixture of the method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion, the sludge used in said preparing steps of the sludge coal mixture is the resulted coal sludge which is formed by such way that the sewage from the sewage treatment plants is adsorbed and filtered by fine coal and calcium oxide so as to remove impurities from the sewage.

Said preparing steps of the sludge coal mixture of the method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion further comprise a step of injecting said sludge coal mixture by a screw pump, from the middle or top of the circulating fluidized bed boiler and the combustion temperature of the circulating fluidized bed boiler is controlled at 850-950° C.

Preferentially, said clean burning steps of the sludge coal mixture in the method for treating sludge of the sewage treatment plants by using circulating fluidized combustion further comprises a step of separating and catching un-burned off coal and waste particles carried out together with the hot flue gas by a fly ash separator installed at the outlet or rear of a combustion furnace of said circulating fluidized bed boiler, and then delivering them back into the dense-phase area of the combustion furnace from a returning device under the separator for the purpose of circulating combustion.

According to another aspect of this invention, there is provided a method for treating sludge of the sewage treatment plants by using circulating fluidized bed boiler combustion, comprising:

a. preparing step of sludge coal mixture by adding the sludge and dispersant into the pea coal, all which are milled in a ball mill into mixed coal slurry of 50-150 μm in granularity, then adding the stabilizing agent and stirring the mixed coal slurry to form the sludge coal mixture; and

b. clean burning step of the sludge coal mixture by feeding the resulted sludge coal mixture into a circulating fluidized bed boiler for clean burning, the furnace of circulating fluidized bed boiler contains media materials composed of quartz sand and limestone.

In said preparing steps of the sludge coal mixture of the method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion, the sludge used in said preparing steps of the sludge coal mixture is the resulted coal sludge which is formed by such way that the sewage from the sewage treatment plants is adsorbed and filtered by fine coal and calcium oxide so as to remove impurities from the sewage.

Said preparing steps of the sludge coal mixture of the method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion further comprise a step of injecting said sludge coal mixture by a screw pump, from the middle or top of the circulating fluidized bed boiler.

Said clean burning steps of the sludge coal mixture in the method for treating sludge of the sewage treatment plants by using circulating fluidized combustion further comprises a step of separating and catching un-burned off coal and waste particles carried out together with the hot flue gas by a fly ash separator installed at the outlet or rear of a combustion furnace of said circulating fluidized bed boiler, and then delivering them back into the dense-phase area of the combustion furnace from a returning device under the separator for the purpose of circulating combustion.

Said method according to this present invention is to prepare the sludge of the sewage treatment plants into the sludge coal mixture, and then delivered them into the circulating fluidized bed boiler for combustion, with the combustion efficiency up to 98%. The inflammable matters contained in such slurry combust in the furnace and then transformed into the heat energy, while other impurities are mixed in the fly ash during the combustion to discharge.

The burn off fly ash contained in the flue gas from the circulating fluidized bed boiler is collected by an electrostatic dust remover or a bag collector 5, with the dust removal rate of up to 99.9%, so that the emission concentration of the dust contained in the flue gas is less than 80 mg/m³, that of SO₂ in the flue gas less than 100 mg/m³, that of NOx less than 150 mg/m³ and Ringelmen blackness of the flue gas lower than class I, all which meet the requirements for atmospheric environmental protection as specified in the national standard. As a result, the environmental pollution caused by fly ash can be avoided completely.

The circulating fluidized bed boiler burning said sludge coal mixture can be of the hot water boiler, or LP, HP or MP steam boiler and the produced steam can be used for power generation, urban heat supply or cogeneration, integrating sludge treatment and power generation or supply, so the energy and environmental protection control expenses are saved in large number.

The fly ash produced during combustion contains silicate and CaSO₄ both in comprehensive utilization mainly for following purposes:

a. With gelatination, the fly ash is the substitute for slag and gypsum to make up the insufficient slag resources and used as the raw materials for preparing the cement, i.e., mixing with the cement clinker in the proportion of 50% to prepare the 325# cement. The fly ash with high CaSO₄ content can be used for preparing special cement and expansion agent.

b. The fly ash is used for making fly ash paving brick, square brick, grass brick and curbstone, etc.

c. The fly ash is used for preparing the fly ash concrete for road and airport.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 indicates a process flow diagram of using urban sewage to prepare sludge coal mixture;

FIG. 2 indicates a process flow diagram of using pea coal and sludge from the sewage treatment plants to prepare sludge coal mixture;

FIG. 3 indicates a process flow diagram of using fine coal and sludge from the sewage treatment plants to prepare sludge coal mixture;

FIG. 4 is a process flow diagram showing that said sludge coal mixture is combusted in a circulating fluidized bed to produce steam and hot water.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of this invention are described as follows with reference to the attached drawings.

Generally, this invention provides a method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion, comprising: a. preparing step of the sludge coal mixture by adding fine coal and dispersant into sludge from the sewage treatment plants, then stirring and adding stabilizing agent; b. clean burning step of the sludge coal mixture, by feeding the prepared sludge coal mixture into the circulating fluidized bed boiler for clean burning, wherein the furnace of the circulating fluidized bed boiler contains fluidized media of quartz sand and limestone.

Now, the method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the present invention will be described as following.

I. Preparation Step of the Sludge Coal Mixture

The method of preparing said sludge coal mixture can be divided into following two according to different processes used: i.e., using the sewage to prepare the sludge coal mixture and using the sludge from the sewage treatment plants to prepare the sludge coal mixture;

{circle around (1)} Using the urban sewage to prepare the sludge coal mixture The sewage and waste water are adsorbed and filtered by fine coal and CaO (calcium oxide) to form the resulted coal sludge which is mixed with others fine coal to form said sludge coal mixture, as shown in FIG. 1 for its process flow.

In detail, the fine coal, coal based activated carbon and CaO are added in a filter tank as the filter materials; the multi-stage filter tank can be constructed as required for the sewage processing quantity. The fine coal and CaO have strong absorption ability and flocculability. When the urban sewage flows through the filter tank or a multi-stage filter tank, most of impurities contained in the urban sewage are filtered and absorbed in filter layers of fine coal, coal based activated carbon and CaO in the filter tank, and the rest impurities and partial fine coal are existed as colloid in the filter water which flows into the settlement basin. The settlement effect due to the flocculation makes the rest lower impurities in the sewage settle together with the fine coal. The clean water in the upper part of the settlement basin flows into a treatment water pond, and the solid and liquid contained in the coal slurry in the lower part of the settlement basin can be at a ratio up to 30-40%, and delivered by the screw pump under the settlement basin into the bottom flow storage tank.

The filter materials composed of fine coal, coal based activated carbon and CaO in the filter tank, after used for a certain period, saturate to form the resulted coal sludge which now can contain up to 30%-40% moisture. The saturated resulted coal sludge in the filter tank is delivered by a screw pump into a bottom flow storage tank which is used as a secondary settlement basin. The clean water in the upper part of the upper flow storage tank is discharged in the treatment water pond, and the lower resulted coal sludge of the storage tank is delivered by the screw pump into a slurry preparing tank.

The filter materials in the filter tank may contain certain quantity of the calcium hydroxide and play roles of denitrification, dephophorization, decoloring and deodorization to the sewage during the filtering.

For the resulted coal sludge delivered into the slurry preparing tank, each batch, before mixing, shall be assayed to inspect the moisture, etc., contained in it, and then mixed with certain quantity of the fine coal in a proportion until the ratio of the coal to the moisture is up to about 6.5-7:3.5-3, and then added with the dispersant. Now an agitator provided on the top of the slurry preparing tank is started up to mix at the rotary speed of 60 r.p.m. The mixture is mixed continuously at the normal temperature and mixed with the dispersant, and then delivered in a coal sludge slurry storage tank after cut by the cutting pump and slaked. Both the dispersant and the stabilizing agent are called additive together. The NDF water coal mixture additive that is developed and produced by the Surface and Interface Chemical Engineering Technology Center of the Nanjing University can be used.

The water in the treatment water pond is pumped into an existing sewage treatment system of the sewage treatment plant for processing again, and after reaching a standard, is discharged or utilized again.

It is necessary for each batch of the sludge coal mixture, before preparing, to test so as to determine proportions and quantities of various components. Said sludge coal mixture in the slurry storage tank shall be mixed 1-2 hours a day as required so as to prevent hard settlement.

{circle around (2)} Using the sludge from the urban sewage treatment plants to prepare the sludge coal mixture

The preparing method can be divided into two types according to different original conditions of the coal used:

a. In the case of using the pea coal to prepare said sludge coal mixture, as shown in FIG. 2 for the process flow, the pea coal is crushed in a crusher, and then weighted and delivered into a ball mill; the sludge in the sludge pool is delivered by a pump in a proportion into the ball mill, and the urban sewage in the sewage storage tank and the additive in an additive storage tank are delivered according to the parts by weight into the ball mill, i.e., 10-20 parts for sludge, 50-60 parts for fine coal, 20-25 parts for water and 2-5 parts for dispersant. The coal is milled into the 50-150 μm mixed coal slurry which is delivered by the screw pump into the slurry preparing tank and then mixed and added with the stabilizing agent. The mixture is cut by the cutting pump and slaked, forming the sludge coal mixture, which is then delivered into a slurry storage tank. Both the dispersant and the stabilizing agent are called additive together. The NDF water coal mixture additive that is developed and produced by the Surface and Interface Chemical Engineering Technology Center of the Nanjing University can be used.

b. In the case of using the fine coal to prepare said sludge coal mixture, as shown in FIG. 3 for the process flow, the fine coal in a fine coal bay, the sludge in a sludge pool, the sewage in a sewage storage tank, and the dispersant in an additive storage tank (same as the above mentioned) are delivered by the delivery equipment according to following parts by weight into the slurry preparing tank, i.e., 10-20 parts for sludge, 50-60 parts for fine coal, 20-25 parts for water and 2-5 parts for dispersant; in addition, the fine coal is 50-150 μm in diameter. The mixture is mixed and added with the stabilizing agent (same as the above mentioned), and cut by the cutting pump and slaked to form the sludge coal mixture, which is then delivered into the slurry storage tank.

Before preparation of each batch of the coal slurry, the coal quality and the sludge must be assayed and the small test must be carried out to determine proportions of coal, sludge, urban sewage, dispersant and stabilizing agent. Said sludge coal mixture in the slurry storage tank shall be mixed 1-2 hours a day as required so as to prevent hard settlement.

II. Clean Burning of the Sludge Coal Mixture

This invention provides retrofit on the circulating fluidized bed boiler 4 in order to adapt to the combustion features of the sludge coal mixture made of the sludge, as shown in FIG. 4 for the process flow.

For the conventional circulating fluidized bed boiler, the dry fine coal is used as the fuel which is delivered by a screw conveyer into the furnace from the bottom of the boiler. The circulating fluidized bed boiler 4 for burning the water coal mixture is structured such that based on the conventional boiler design a sludge coal mixture inlet is set up on the top or in the middle of the boiler. If the boiler 4 only burns the sludge coal mixture, the screw coal for feeding equipment will be removed, and if it burns the coal and the sludge coal mixture, the screw coal for feeding equipment will be retained.

The circulating fluidized bed boiler 4 for burning the sludge coal mixture is ignited in the manner of hot air flow pattern, in which the diesel oil is atomized by an ignition gun and then burns in a pre-combustion tube; the produced high temperature flue gas enters the isobaric air chamber to heat the air in the air supply duct, and then the heated high temperature air enters a fluidized on-bed material layer through an air distribution board. When the materials on the bed is heated over 450° C., a little of the sludge coal mixture will be added in a circulating fluidized bed boiler, and now, the boiler rises quickly in temperature. When the temperature in the boiler 4 reaches 600° C., slurry supply can be increased gradually, the boiler's temperature rises quickly and said sludge coal mixture is in normal combustion; now, oil supply is interrupted and the pre-combustion chamber is shut off to complete the ignition; the supply of sludge coal mixture is increased to adjust the air volume required for operation of the blower 3 and the induced draft fan 7.

The sludge coal mixture is supplied to the circulating fluidized bed boiler 4 at high level; there are two or three slurry inlets in the middle of the medium and large boilers of 35 tons or over 29 MW, from which the sludge coal mixture is delivered into a furnace; also the sludge coal mixture can be delivered from the top of these medium and small boilers, and this way sufficiently utilizes the boiler's height to accelerate separation of the moisture and volatile matters contained in the sludge coal mixture, in favor of the combustion.

The sludge coal mixture is delivered by a feeding slurry pump 3 of the boiler from the slurry storage tank 1 to the circulating fluidized bed boiler 4. Through the slurry inlet, the sludge coal mixture enters the circulating fluidized bed boiler and then becomes drops which will become smaller particles due to disturbance of the fluidized hot air flow in the furnace and then drop downwards in the furnace at 850-950° C. During the drop, the sludge coal mixture drops are heated and decomposed, start burning, and then enter the fluidized bed composed of quartz sand and limestone. During continuous heating of the broiling materials on the fluidized bed, they are removed the moisture and violate matters quickly, and then subject to combustion and coke combustion. The further decomposed fine particles of the granular sludge coal mixture agglomerates under the fluidized state are carried out of the dense-phase area together with the hot flue gas and then enter the suspension chamber for further burning. There are fly ash separation and returning devices installed at the outlet or rear of a combustion furnace of the furnace. The media materials and large un-burned off coal and waste particles all which are carried out together with the hot flue gas are separated by a separator, and then delivered back into a dense-phase area of the combustion furnace from a returning device under the separator for the purpose of circulating combustion, with the combustion efficiency up to 98%.

The sludge coal mixture makes low temperature combustion in the furnace at 850-950° C. so as to effectively restrain generation of thermal NOx during combustion of the fuel. The quartz sand and the limestone are used as the fluidized media to add into the furnace; the limestone is decomposed into CaO at high temperature, which acts with SO₂ produced after combustion of the sulfur contained in the fine coal in the sludge coal mixture to generate CaSO₄. The action: SO₂+CaO+1/2O₂=CaSO₄, restrains emission of SO₂ in the flue gas, resulting in direct desulfurization in the circulating fluidized bed boiler. The limestone (desulfurating agent) added into such boiler is about 1-2 mm in size, and the mol ratio of calcium to sulfur is equal to 1.5-2.5 at the desulfurization efficiency of up to 85%-95%. The inflammable matters in said sludge coal mixture burn in the furnace and then are transformed into the heat energy; during combustion, other impurities are mixed in the fly ash to emit.

The burn off fly ash contained in the flue gas from the circulating fluidized bed boiler is collected by an electrostatic dust remover or a bag dust remover 5, with the dust removal rate of up to 99.9%, so that the emission concentration of the dust contained in the flue gas is less than 80 mg/m³, that of SO₂ in the flue gas less than 10 mg/m³, that of NOx less than 150 mg/m³ and Ringelmen blackness of the flue gas lower than class I, all which meet the requirements for atmospheric environmental protection as specified in the national standard. In addition, the flue gas is emitted from the stack 6.

The circulating fluidized bed boiler 4 can be of a hot water boiler, or LP (low pressure), HP (high pressure) or MP (medium pressure) steam boiler and the produced steam can be used for power generation, urban heat supply or cogeneration, integrating sludge treatment and power generation or supply, so the energy and environmental protection control expenses are saved in large number.

The fly ash collected by the dust remover 5 after combustion of the sludge coal mixture can be utilized comprehensively. The fly ash produced after clean combustion of the sludge coal mixture in the circulating fluidized bed boiler 4 is collected by the dust remover 5, and then delivered through an enclosed delivery device into the enclosed ash silo 8. Stacked to a certain quantity, the fly ash will be transported away by the dedicated vehicles for fly ash. The environmental pollution caused by fly ash is avoided completely. The fly ash contains silicate and CaSO₄ both in comprehensive utilization mainly for following purposes:

a. With gelatination, the fly ash is the substitute for slag and gypsum to make up the insufficient slag resources and used as the raw materials for preparing the cement, i.e., mixing with the cement clinker in the proportion of 50% to prepare the 325# cement. The fly ash with high CaSO₄ content can be used for preparing special cement and expansion agent.

b. The fly ash is used for making fly ash paving brick, square brick, grass brick and curbstone, etc.

c. The fly ash is used for preparing the fly ash concrete for road and airport.

EXAMPLE 1

An urban sewage treatment plant is taken as an example. This plant lies on the border of the city. The sludge in its sewage mainly contains the organic substances and the phosphorus, and the sewage is treated and utilized comprehensively by using the circulating fluidized combustion.

The coal sludge resulting from absorption, settlement and filtering of the urban sewage by using the fine coal and CaO is mixed with the fine coal to form the sludge coal mixture. The fine coal and CaO in 50-150 μm are added into the filter tank which is in three stages. The fine coal and CaO have strong absorption and filtering abilities. When the urban sewage flows through the three-stage filter tank, most of organic substances and impurities contained in the urban sewage are filtered and absorbed in the fine coal, CaO and filter layer in the filter tank, and the rest impurities and partial fine coal exist as colloid in the filter water which flows into the settlement basin. The settlement effect due to the flocculation effect makes the rest lower impurities in the sewage settle together with the fine coal. The clean water in the upper part of the settlement basin flows into a treatment water pond, and the resulted coal sludge in the lower part of the settlement basin can contain up to 30-40% moisture, and is delivered by the screw pump under the settlement basin into the bottom flow storage tank.

The filter materials composed of fine coal and CaO in the filter tank, after used for a certain period, are saturated to form the resulted coal sludge which now can contain up to 30%-40% moisture. The saturated resulted coal sludge in the filter tank is delivered by the screw pump into the bottom flow storage tank which is used as a secondary settlement basin. The clean water in the upper part in the upper flow storage tank is discharged in the treatment water pond, and the lower resulted coal sludge in the storage tank is delivered by the screw pump into the slurry preparing tank.

The filter materials in the filter tank contain certain quantity of the calcium hydroxide and play roles of denitrification, dephophorization, decoloring and deodorization to the sewage during the filtering.

The fine coal in 29.3 MJ (MJ referred to abbreviation of megajoule, the thermal unit)/Kg, with ash content≦10% and 50-150 μm in size is selected. A certain quantity of such fine coal is mixed until the ratio of the coal to the moisture is up to about 6.5-7:3.5-3, and then the dispersant accounting for 3% in the total is mixed. Now the agitator on the top of the slurry preparing tank is started up to mix at the rotary speed of 60 r.p.m. The mixture is mixed continuously at the normal temperature and then mixed with the dispersant in the same quantity as the above, and after this, the mixture is delivered into the coal sludge slurry storage tank after cut by the cutting pump and slaked. Both the dispersant and the stabilizing agent are called additive together. The NDF water coal mixture additive that is developed and produced by the Surface and Interface Chemical Engineering Technology Center of the Nanjing University can be used.

Said sludge coal mixture in the storage tank shall be mixed 1-2 hours a day so as to prevent hard settlement. Two 75t/h sludge coal mixture steam boilers of circulating fluidized bed are installed. There are two slurry inlets in the middle of the front wall of the boiler so as to delivery said sludge coal mixture into the furnace.

The circulating fluidized bed boiler burning the sludge coal mixture is ignited in the hot air flow pattern, in which the diesel oil is atomized by an ignition gun and then bums in the pre-combustion tube; the produced high temperature glue gas enters the isobaric air chamber to heat the air in the air supply duct, and then the heated high temperature air enters the fluidized on-bed material layer through air distribution boards. When the materials on the bed is heated over 450° C., a little of the sludge coal mixture will be added in the circulating fluidized bed boiler, and now, the boiler rises quickly in temperature. When the temperature in the boiler reaches 600° C., slurry supply can be increased gradually, the temperature in the circulating fluidized bed boiler rises quickly and said sludge coal mixture is in normal combustion; now, oil supply is interrupted and the pre-combustion chamber is shut off to complete the ignition; the supply of sludge coal mixture is increased to adjust the air volume required for operation of the blower 3 and the induced draft fan 7.

Through the sludge coal mixture inlet, the sludge coal mixture enters the circulating fluidized bed boiler and then becomes drops which will become smaller particles due to disturbance of the fluidized hot air flow in the furnace and then drop downwards in the furnace at 850-950° C. During the drop, the sludge coal mixture drops are heated and decomposed, start burning, and then enter the fluidized bed composed of quartz sand and limestone. During continuous heating of the broiling materials on the fluidized bed, the moisture and volatile matters are removed quickly therefrom, and then the broiling materials are subjected to combustion and cake combustion. The further decomposed fine particles of the granular sludge coal mixture agglomerates under the fluidized state are carried out of the dense-phase area together with the hot flue gas and then enter the suspension chamber for further burning. There are fly ash separation and returning devices installed at the outlet or rear of the combustion furnace. The media materials and large un-burned off coal and waste particles all which are carried out together with the hot flue gas are separated by the separator, and then delivered back into the dense-phase area of the combustion furnace from a returning device under the separator for the purpose of circulating combustion, with the combustion efficiency up to 98%.

The sludge coal mixture makes low temperature combustion in the furnace at 850-950° C. so as to effectively restrain generation of thermal NOx during combustion of the fuel. The quartz sand and the limestone are used as the fluidized media to add into the furnace; the limestone is decomposed into CaO at high temperature, which acts with SO₂ produced after combustion of the sulfur contained in the fine coal in the sludge coal mixture to generate CaSO₄. The action: SO₂+CaO+1/2O₂=CaSO₄, restrains emission of SO₂ in the flue gas, resulting in direct desulfurization in the circulating fluidized bed boiler. The limestone (desulfurating agent) added into the circulating fluidized bed boiler is about 1-2 mm in size, and the mol ratio of calcium to sulfur is equal to 1.5-2.5 at the desulfurization efficiency of up to 85%-95%. The inflammable matters in said sludge coal mixture burn in the furnace and then are transformed into heat energy; during combustion, other impurities are mixed in the fly ash to emit.

Two boilers burn 21-25 t sludge coal mixture per hour and produce 150 t steam in 3.9-3.5 Mpa per hour, resulting in about 20000 KW power generation. The burn off fly ash is contained in the flue gas from the circulating fluidized bed boiler and collected by an electrostatic dust remover or a bag dust remover, with the dust removal rate of up to 99.9%, so that the emission concentration of the dust contained in the flue gas is 50 mg/m³, that of SO₂ in the flue gas is 80 mg/m³, that of NOx is 90 mg/m³ and Ringelmen blackness of the flue gas is lower than class I.

EXAMPLE 2

The same urban sewage treatment plant is taken as an example. This plant lies on the border of the city. The sludge in its sewage mainly contains the organic substances and the phosphorus. The sludge during the sewage treatment is treated and utilized comprehensively by using the circulating fluidized combustion.

Two 75t/h sludge coal mixture steam boilers of circulating fluidized bed are installed. The pea coal in 29.3 MJ/Kg, with ash content≦10%, is selected. The mixture prepared in the proportion of 60% coal, 15% sludge, 22% sewage and 3% dispersant is weighted by a metering device and then delivered into the ball mill for wet milling up to forming the 50-150 μm mixed coal slurry which is delivered by a screw pump into the slurry preparing tank and mixed by a agitator at the rotary speed of 60 r.p.m. The mixture is added with the dispersant and after cut by a cutting pump and slaked, prepared into the sludge coal mixture which is delivered into the slurry storage tank. Said sludge coal mixture shall be mixed 1-2 hours a day regularly and delivered into the sludge coal mixture steam boiler of circulating fluidized bed. Two boilers burn 21-25 t sludge coal mixture per hour and produce 150 t steam in 3.9-3.5 Mpa per hour, resulting in about 20000 KW power generation. The flue gas after combustion is dedusted by an electrostatic dust remover to emit. The boiler's dust removal rate, the emission concentration of the injurants contained in the flue gas and the Ringelmen blackness of the flue gas all meet the atmospheric environmental protection requirements as specified in the national standard.

Some preferred embodiments of this invention have been illustrated and described with reference to the drawings thereof, and also the special applications and explanatory embodiments of this invention are stated and discussed, laying a foundation for embodiment of this invention in various modes. There are still many variants within the scope of this invention. It will be understood by these persons skilled in the art that various change may be made therein without departing from the spirit and scope of this invention as defined by the appended claims of this invention and their equivalent thereof. 

1. A method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion, comprising: preparing step of sludge coal mixture by adding fine coal and dispersant into sludge from a sewage treatment plants, then stirring and adding stabilizing agent; and clean burning step of the sludge coal mixture by feeding the prepared sludge coal mixture into a circulating fluidized bed boiler for clean burning, wherein a furnace of the circulating fluidized bed boiler contains fluidized media of quartz sand and limestone.
 2. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 1, wherein in said preparing steps of the sludge coal mixture, the part by weight of the components of the sludge coal mixture is such as 10-20 parts for sludge, 50-60 parts for fine coal, 20-25 parts for water and 2-5 parts for dispersant and the fine coal is 50-150 μm in diameter.
 3. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 1, wherein the sludge used in said preparing steps of the sludge coal mixture is the resulted coal sludge which is formed by such way that the sewage from the sewage treatment plants is adsorbed and filtered by fine coal and calcium oxide so as to remove impurities from the sewage.
 4. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 1, wherein in the clean burning steps of said sludge coal mixture, further comprising a step of injecting said sludge coal mixture, by a screw pump, from the middle or top of the circulating fluidized bed boiler.
 5. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 4, wherein the combustion temperature of the circulating fluidized bed boiler is controlled at 850-950° C.
 6. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 5, wherein in the clean burning steps of said sludge coal mixture, further comprising a step of separating and catching un-burned off coal and waste particles carried out together with the hot flue gas by a fly ash separator installed at the outlet or rear of a combustion furnace of said circulating fluidized bed boiler, and then delivering them back into the dense-phase area of the combustion furnace from a returning device under the separator for the purpose of circulating combustion.
 7. A method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion, comprising following steps: preparing step of sludge coal mixture by adding the sludge and dispersant into the pea coal, all which are milled in a ball mill into mixed coal slurry of 50-150 μm in granularity, then adding the stabilizing agent and stirring the mixed coal slurry to form the sludge coal mixture; and clean burning step of the sludge coal mixture by feeding the resulted sludge coal mixture into a circulating fluidized bed boiler for clean burning, the furnace of circulating fluidized bed boiler contains media materials composed of quartz sand and limestone.
 8. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 7, wherein in the clean burning steps of said sludge coal mixture, further comprising a step of injecting said sludge coal mixture, by a screw pump, from the middle or top of the circulating fluidized bed boiler.
 9. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 7, wherein in the clean burning steps of said sludge coal mixture, further comprising a step of injecting said sludge coal mixture, by a screw pump, from the middle or top of the circulating fluidized bed boiler.
 10. The method for treating sludge of the sewage treatment plants by using the circulating fluidized bed combustion according to the claim 9, wherein in the clean burning steps of said sludge coal mixture, further comprising a step of separating and catching un-burned off coal and waste particles carried out together with the hot flue gas by a fly ash separator installed at the outlet or rear of a combustion furnace of said circulating fluidized bed boiler, and then delivering them back into the dense-phase area of the combustion furnace from a returning device under the separator for the purpose of circulating combustion. 